Fluid operated actuator for movable members

ABSTRACT

A fluid operated actuator for controlling the movement of a member such as a valve and being connectable into a fluid circuit including pressure operated means for indicating extreme operative positions of the aforementioned member. The actuator including a piston-cylinder assembly, the piston of which is connectable to the aforementioned member, and pressure relief means operative to prevent the pressure acting on the piston reaching a predetermined level which is below that necessary to operate the indicating means. Cut-out means is provided within the piston-cylinder assembly to render the relief means inoperative when the piston reaches or approaches positions corresponding to the aforementioned extreme operative positions of the associated member, so that the pressure acting on the piston is able to rise sufficiently to operate the indicating means.

United States Patent (Iusveller 1 Sept. 5, 1972 [54] FLUID OPERATED ACTUATOR FOR 2,711,797 6/1955 Muller ..9l/40l MOVABLE MEMBERS 3,018,762 l/1962 Korb ..91/ l 70 Inventor: J C u Moonee Ponds, FOX Australia Primary Examiner-Martin P. Schwadron [73] Asslgnee: f' Rand Assistant Examiner-Irwin C. Cohen Manbymong, Vwwfla, Ausflalla- Attorney-Billet, Brown, Ramik & Holt- [22] Filed: Sept. 18, 1970 I [21] Appl. No.: 73,508 [57] ABSTRACT A fluid operated actuator for controlling the movement of a member such as a valve and being connecta- [30] Forelgn Apphcamn Pnonty Data ble into a fluid circuit including pressure operated Sept. 25, 1969 Australia ..61,402/69 means for indicating extreme operative positions of the aforementioned memben'The actuator-including a [52] US. Cl. 491/420, 91/422, 91/437 piston-cylinder assembly, the piston of which is con- [51] Int. Cl ..Fl5b 13/042 ne table to the aforementioned member, and pressure Field 0t Search 222, relief means operative to prevent the pressure acting 91/420 437; 92/65; 137/596-12 595-13 on the piston reaching a predetermined level which is below that necessary to operate the indicating means. [56] References Clted Cut-out means is provided within the piston-cylinder UNITED STATES PATENTS assembly to render the relief means inoperative when 7 the piston reaches or approaches positions cor- I Presnell X responding to the aforementioned extreme operative Hoffman positions of the associated member so that the pres- Chatflet ure acu'ng on the piston able to rise sumciently to 2,378,409 6/1945 Joy ..91/422 operate the indicating means 2,810,930 10/1957 MacDonald et al. ..91/400 2,735,502 2/1956 Muller ..91/401 40 Claims, 24 Drawing figures mimosa? 5.912 6 5 5 SHEET 010! 11 Q e R;

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/ mvemon JOHN CUSVELLER magnum PATENTED E! 5 I SHEET llUF 11 \NVENTDR JOHN CUSNELLER FLUID OPERATED ACTUATOR FOR MOVABLE This invention relates to fluid operated actuators for movable members, and is particularly concerned with such actuators for use in actuator systems which include means for indicating when the connected member approaches or reaches a desired position.

There are numerous movable member installations in which the member is either hidden or remotely located such that means is required to indicate whether the actuator has moved that member into or towards a particular position. An example installation requiring such indicator means is a valve installation on board a boat or ship, and it will be convenient to hereinafter describe the invention in relation to such an installation.

In view of the remote location of numerous valves on board ships, it is desirable if not essential to have some means for indicating whether each valve is in its open or closed position. It is common to control the movement of such valves through fluid pressure operated actuators, and the indicator system is usually formed separate to the actuator system although it may also be fluid controlled. The result is a complex and expensive maze of pipe-work, conduit, and ancillary equipment. In addition, known indicator systems employing fluid pressure are often adversely affected by temperature changes, and may give an incorrect indication if the associated valve is stuck or encounters abnormal resistance between the open and closed positions.

It is a principal object of the present invention to provide a movable member actuator which is operated by fluid pressure and can be incorporated into a system having means for indicating whether the actuator has reached or is approaching a particular operative condition. It is a further object of the invention to provide such an actuator which is relatively simple in construction and capable of being used with indicator means which is not susceptible to normal temperature variations. Yet another object of the invention is to provide such an actuator system incorporating indicator means which provides a positive indication only when the actuator adopts a predetermined operative condition.

In an actuator system including an actuator according to the invention, the pressurized fluid causing operative movement to the actuator is also utilized to operate the indicator means. As a result, an installation including such an actuator is much simpler than similar installations incorporating conventional actuators and indicator means. Basically the actuator includes pressure relief means which is operative to prevent the fluid pressure in at least part of the actuator system reaching a predetermined indicator pressure, and cut-out means for rendering the relief means inoperative when the actuator adopts a particular operative condition.

When an actuator according to the invention is applied to a marine valve, it is generally convenient to have the actuator arranged to indicate both the open and closed positions of that valve. The following particular embodiments of the actuator will be described as so arranged, but it is to be realized that in some cases indication of a single position will be sufircient. Furthennore, it must be understood that the invention is not restricted to actuation of valves, but is applicable to actuation of almost any other movable member such as doors, gates and hoists, to name but a few.

The following description refers in more detail to these essential features and further optional features of the invention. To facilitate an understanding of the invention, reference is made to the accompanying drawings where these features are illustrated in preferred form. It is to be understood however, that the essential and optional features of the invention are not limited to the specific forms of these features as shown in the drawings.

In the drawings:

FIG. 1 is a partially sectioned side elevational view of an actuator according to the invention, showing the actuator between its two extreme operative positions;

FIG. 2 is an end view of the actuator shown in FIG. 1 in the direction of arrow A;

FIG. 3 is an enlarged transverse cross-sectional view taken along line III-III of FIG. 1, with parts omitted for convenience of illustration, and showing the control valves as positioned during closing movement of the actuator;

FIG. 4 is a cross-sectional view taken along line LV--LV of FIG. 3; 7

FIG. 5 is a cross-sectional view taken along line V- V of FIG. 3;

FIG. 6 is an enlarged longitudinal cross-sectional view of the piston assembly of the actuator of FIG. 1, showing that assembly approaching the position in which a connected valve member is almost closed;

FIG. 7 is a transverse cross-sectional view taken along line VIIVII of FIG. 6;

FIG. 8 is a sectional view taken along line VIII-VIII of FIG. 6;

FIG. 9 is a view similar to FIG. 6 but showing the piston assembly location when the connected valve member is closed;

FIG. 10 is a view similar to part of FIG. I, but showing the piston assembly location when return movement of the assembly is initiated to cause opening movement of the. connected valve member;

FIG. 11 is a view similar to FIG. 3 but showing the control valves as positioned during opening movement of the actuator;

FIG. 12 is a view similar to FIG. 6 but showing the piston assembly as located when the connected valve member is approaching its fully open position;

FIG. 13 is a view similar to FIG. 9 but showing the piston assembly location when the connected valve member is fully opened;

FIG. 14 is a view similar to FIG. 3 but showing the control valves in the closed position as adopted when the actuator is inoperative;

FIG. 15 is an enlarged cross-sectional view taken along line XVXV of FIG. 2;

FIG. 16 is an enlarged cross-sectional view taken along line XVI-XVI of FIG. 2;

FIG. 17 is a diagrammatic view of an actuator according to FIG. 1 connected into a fluid circuit;

FIG. 18 is a side elevational view of an alternative embodiment of an actuator according to the invention;

FIG. 19 is a side elevational view of the actuator shown in FIG. 18 taken in the direction of arrow B, and partially sectioned for convenience of illustration;

FIG. 20 is a cross-sectional view taken along line XX-XX of FIG. 18;

FIG. 21 is a cross-sectional view taken along line XXIXXI of FIG. 19, parts being omitted for convenience of illustration;

FIG. 22 is a cross-sectional view taken along line XXII-XXII of FIG. 19; V

FIG. 23 is a cross-sectional view taken along line XXIII-XXIII of FIG. 19; and

FIG. 24 is a crossrsectional view taken along line XXIV-XXIV of FIG. 23.

FIGS. 1 to 16 of the drawings show one particular embodiment of the invention for use with slidable gate or other linear movement valves, in which the actuator includes a primary piston 2 and a secondary piston 3, both of which are mounted within a single cylinder 4 for axial movement relative thereto. The primary piston 2 divides the cylinder 4 transversely into two chambers 5 and 6, and for convenience those chambers will be hereinafter called opening and closing chambers respectively. A piston rod 7 is secured to the secondary piston 3 for movement therewith and extends through end wall 8 of the cylinder 4 for connection to a valve member 9 (see FIG. 17), and it is preferred that the opening chamber 5 isdefined between the end wall 8 and the primary piston 2.

The end wall 8 may form part of an end cap 11 secured to the cylinder body 12 and having a fluid passageway 13 through a barrel portion 14 thereof and connecting at one end with the opening chamber 5. Alternatively however, the fluid passageway 13 may be formed through the side wall of the cylinder body 12. A conduit 15 may be connected to the fluid passageway 13 and connected into a fluid circuit of the actuator as hereinafter described. The bore of the cap 11 through which the piston rod 7 passes is preferably defined in part by a removable bush 16 having any suitable means 17 therein to form a substantially fluid-tight seal with thepiston rod 7 whilst allowing axial movement of that rod.

A cap member 18 may also close the opposite end of the cylinder 4, and that member preferably has a control valve block 19 formed integral therewith (see FIGS. 2 to 5), although the block 19 may be formed as a separate element and connected to any other appropriate part of the actuator as desired. In the example arrangement shown, the valve block 19 includes two valve compartments 21 and 22, each of which contains a spring'influenced one-way valve 23. The compartment 21 is in direct communication with the cylinder chamber 6 through a passage 24, and compartment 22 communicates with the conduit 15 through a passage 25 and is thereby connected with the cylinder chamber 5. It will be appreciated that the external conduit 15 may be substituted by some other means such' as a passage formed within the wall of the cylinder body 12.

An intermediate compartment 26 may be formed within the block 19 and contains an axially slidable spool element 27 arranged to open either valve 23 as hereinafter described. Ports 28 and 2 9 communicate with the compartment 26 on opposite sides respectively of the spool element 27 as shown in FIG. 3, and each serves as inlet and outlet means for a respective one of the chambers 5 and 6. When the actuator is connected into a fluid system, a supply-exhaust conduit 31 is connected to port 28, and a similar conduit 32 is connected to the port 29. The spool element 27 serves as a fluid seal between the ports 28 and 29.

Each valve 23 preferably includes a conical nose section 33 which normally projects into and closes a valve orifice 34. The spring setting of each valve 23 is such that the valve may be opened by the normal operating pressure of the fluid used to operate the actuator. A

tothe secondary piston 3 and the piston rod 7 between two limit stops each of which is preferably secured to the secondary piston 3. In the arrangement shown in FIGS. 1 and 6, one such stop is formed by an annular nut 36 secured to the terminal end portion of the secondary piston 3, and the other stop is formed by a collar 37 secured to the secondary piston 3 adjacent the piston rod 7. It is convenient to form the collar 37 integral with the secondary piston 3. Although the secondary piston 3 is shown integral with the piston rod 7, it will be appreciated that thosetwo parts may be formed separate and attached by a'screw thread or other connection. 7

As shown particularly in FIG. 6, the primary piston 2 includes an inner sleeve formed of two parts 38 and 39 which are slidable on the secondary piston 3 between nut 36 and collar 37, and an outer sleeve 41 which is slidable on the inner sleeve parts 38 and 39. The inner sleeve is made in, two parts as a matter of convenience for assembly with the outer sleeve 41, and each part 38 and 39 has an enlarged end portion 42 and 43 respectively which defines an annular shoulder 44 and 45 respectively. The two shoulders 44 and 45 are opposed and each provides a combined stop and sealing surface for cooperation with the outer sleeve 41 as hereinafter described.

The pressure relief means preferably includes a pair of relief valves 46 and 47 which are arranged to prevent the fluid pressure within the cylinder chambers 5 and 6 respectively from reaching the aforementioned indicator pressure that is, when the relief means is not rendered inoperative by the cut-out means. Although the relief valves 46 and 47 may be located externally of the cylinder 4, it is preferred to locate them within a respective relief passage 48 and 49 fonned within the secondary piston 3. Each relief passage 48 and 49 may extend axially of the piston rod 7, and have laterally extending entrance portions 51 and 52 respectively through which communication is made with the respective chambers 5 and 6. The valve 46 exhausts to chamber 6 through a passage 53, and valve 47 exhausts to chamber 5 through passage 54. Obviously, an arrangement may exist in which the two relief valves 46 and 47 are located within a single relief passage.

Each of the relief valves 46 and 48 may include a valve member 55 movable axially relative to the associated relief passage, and a cooperable valve seat 56. In one form as shown in FIG. 6, each valve member 55 is a ball which is locatable within an orifice 56 defining its respective valve seat, and the valve member is preferably biased by a spring 57 towards that seat. Each spring 57 acts on its respective valve ball 55 through a slide member 58 which is provided with flat surfaces 59 (see FIG. 8) or other clearance means or apertures to allow passage of fluid thereby.

The seat 56 of valve 47 may be formed by the bore of a bush 61 mounted within the secondary piston 3, and the axial position of the bush 61 is preferably adjustable to enable variation of the compression of valve spring 57 and consequently the valve closing force. The seat 56 of valve 46 may be formed by one end of the relief passage 48, and the passage 53 is preferably formed as the bore of a bush 62 adjustably mounted in the secondary piston 3 to enable variation of the compression of the spring 57 for the valve 46.

The pressure relief means may also include a pair of passages 63 and 64 formed through respective parts 38 and 39 of the primary piston inner sleeve, and they are located to communicate with the lateral passages 52 and 51 respectively. That communication may be maintained over the length of movement of the primary piston 2 relative to the secondary piston 3, through internal grooves 65 and 66 of the inner sleeve parts 38 and 39 respectively (see FIG. 7).

Cut-out means for the arrangement so far described preferably includes a pair of abutments 67 and 68 (see FIG. 1) which limit axial movement of the primary piston 2 with the secondary piston 3. The abutments 67 and 68 are located adjacent respective opposite ends of the cylinder body 12, and each is arranged to stop movement of the primary piston 2 towards that end whilst allowing continued movement of the secondary piston 3. In one form, the abutment 67 is defined by the axially inner end face of the barrel portion 14 of the end cap 11, and abutment 68 is defined by the axially inner end face of a barrel portion 69 of the end cap 18. The secondary piston is receivable within the bore of each of the barrel portions 14 and 69 to allow the aforementioned continued movement. Obviously, any other suitable abutment may be used such as a simple pin projecting radially inwardly from the side wall of the cylinder 4, or the cross-sectional area of the cylinder body 12 may be reduced to form appropriate stop shoulders.

Safety means is preferably provided within the valve block 19 to prevent the pressure within either chamber 5 and 6 reaching a predetermined upper safe limit. In this regard, the control valves 23a and 23b remain closed when the actuator is not in operation, as shown in FIG. 14, and consequently fluid is trapped in both chambers 5 and 6. That fluid may expand with a rise in ambient temperature, and the resulting increase in pressure may cause damage to the actuator if not relieved. One such safety means is two spring influenced one-way valves 71 and 72 (see FIGS. and 16) which communicate with the chambers 5 and 6 respectively.

Safety valve 71 includes a valve element 73 contained within a chamber 74 and biased to normally close the orifice 75. The orifice 75 communicates direct with the port 25 through a passage 76 to allow fluid under pressure to be exhausted from the chamber 5 by way of conduit 15. The valve chamber 74 communicates with the port 29 through a passage 77, whereby fluid exhausted through the valve 71 is able to drain to a tank or reservoir by way of conduit 32.

The safety valve 72 also comprises a valve element 78 contained within a chamber 79 and biased to normally close an orifice 81. The orifice 81 communicates direct with the passage 24 through a passage 82 to allow fluid under pressure to be exhausted from the chamber 6 by way of the passage 24. The valve chamber 79 communicates with the port 28 througha passage 83, whereby fluid exhausted through the valve 72 is able to drain to a tank or reservoir by way of the piston rod 7. v

One or both of the primary piston abutments 67 and 68 may be adjustable in the axial direction of the cylinder 4 to enable variation of the stop position or positions of the piston 2. Conduits 31 and 32 are selectively connectable to a source of pressurized fluid through a directional control valve 85. In one position of the valve 85, conduit 31 serves to supply fluid to chamber 5, and conduit 32 functions as a drainpipe for chamber 6. In another position of the valve 85, conduit 32 feeds chamber 6, and conduit 31 drains chamber 5. Alternatively, a separate feed and drainpipe may be connected to each chamber and controlled by appropriate valving as required.

A pressure sensitive switch 86 is preferably connected to each of the conduits 31 and 32, and each of the switches 86 may be operatively connected to a suitable indicator device (not shown). Each indicator device may be arranged to produce a visual and/or audible signal, but it is generally convenient to employ simple incandescent lamps as the indicator devices.

Chokes 87 and an accumulator 88 may be provided in the circuit as required according to known procedures and techniques.

The actuator may be hydraulically or pneumatically operated, but hydraulic operation is generally preferred.

Assuming the gate valve 84 is in an open position, the slide 9 may be moved towards the closed position by causing pressurized fluid to enter the actuatorcircuit through conduit 32 which feeds that fluid into the cylinder chamber 6 through port 29. Fluid passing through port 29 enters the intermediate compartment 26, and the pressure thereof opens the valve 23b (FIG. 3) so that the fluid is then able to pass from valve compartment 21 to the chamber 6 through passage 24. That same fluid pressure urges the spool element 27 to open the valve 23a by engagement between the stem 35a and nose section 33a as shown in FIG. 3, and consequently chamber 5 is able to exhaust through conduit 15 to port 28 and conduit 31. In normal operation, that fluid pressure is insufficient to open the relief valve 47 so that the fluid functions to urge the primary and secondary pistons 2 and 3 towards the end cap 11 of the cylinder 4. When the fluid is first introduced into the chamber 6 however, the primary piston 2 moves relative to the secondary piston 3 away from the end cap 18 and subsequently engages the stop collar 37 (see FIG. 1), after which the two pistons 2 and 3 move together with the ters an obstacle such as to increase the resistance to further closing movement, or prevent such further movement, a build-up of pressure will naturallyoccur within the chamber 6. The relief valve 47 is arranged to open before that pressure reaches the aforementioned indicator pressure which is effective to operate the pressure sensitive switch 86 connected to theconduit 32. Thus, fluid is able to escape from the chamber 6 through the passages 49 and 54 to the chamber 5, from where it is discharged through the conduits l and 31.

Assuming closing movement of the actuator continues, a condition is eventually achieved in which the outer sleeve 41 of the primary piston 2 engages and-is stopped by the abutment 67 as shown in FIG. 6. The inner sleeve parts 38 and 39 and secondary piston 3 are able to continue moving together towards the end cap 11, until the shoulder 44 engages the adjacent end of the sleeve 41 as shown in FIG. 9. In this position,'communication between chamber 6 and the relief valve 47 is closed, and the pressure within the chamber 6 acts on the secondary piston 3 alone so that the piston rod 7 continues to move in the closing direction and the stop collar 37 is progressively moved into the bore of the barrel portion 14. Thus, when the'valve slide 9 closes,

pressure is'built up within the chamber 6, and condirectional control valve 85 is operated so that pressure fluid is introduced to the chamber 5 through the conduit 31, and the conduit 32 connects the chamber 6 to drain. It will be appreciated that the closing force of the actuator during the last part of the closing movement is dependent upon the axial end area of the secondary piston 3, since both sleeves of the primary piston 2 are held by the abutment 67, and that area is preferably substantially less than the combined end face area of the primary and secondary pistons which is subjected to fluid pressure within the chamber 5. Thus, for the same fluid pressure, the opening force of the actuator is greater thanthe final closing force thereby ensuring effective operation of the actuator.

When pressurized fluid is fed into the valve block compartment 26 by way of conduit 31 and port 28, valve 23a opens as shown in FIG. 11 to allow the fluid to enter port 25 from where it passes to the chamber 5 by way of the conduit 15. The fluid pressure in compartment 26 also moves the spool element 27 into a position such that the stem b engages the valve nose section 33b and causes the valve 23b to open as shown in FIG. 1 l, and thereby allow the chamber 6 to exhaust through passage 24 to the port 29 and connected conduit 32.

Initial entry of pressurized fluid into the chamber 5 causes the primary piston 2 to adopt the position shown in FIG. 10 in which the inner sleeve part 38 engages the nut 36 and the outer sleeve 41 engages the shoulder 44. After that position has been reached, the primary and secondary pistons move together towards the end cap 18 thereby causing the connected valve 84 to open. Also in this position of the pistons 2 and 3, the relief passage 48 is in communication with the chamber 5 through the lateral entrance 51 and the passage 64 as shown in FIGS. 10 and 12.

During opening movement, pressure build-up within the chamber 5 is relieved in the same manner as described in relation to the chamber 6 during the closing movement, except that the pressure relief is effected through opening of the relief valve 46 with consequent discharge of fluid to the chamber 6 through the passage 53. Fluid escapes from the chamber 6 through the passage 24 and port 29 to the conduit 32 which is connected to drain during this particular operation, as previously described. It will be appreciated that under normal conditions, the pressure in the chamber 5 will not rise to the relief pressure unless abnormal resistance to the opening movement of the valve slide 9 is encountered.

As the valve slide 9 approaches the fully opened position, the outer sleeve 41 of the primary piston 2 engages the abutment 68 as shown in FIG. 12, and is thereby held against further movement in the opening direction. The inner sleeve parts 38 and 39 and secondary piston 3 however, continue to move in that direction as fluid pressure within the chamber 5 acts against their combined effective end area. The shoulder 45 of the inner sleeve part 39 is thereby moved into engagement with the adjacent end of the sleeve 41 as shown in FIG. 13, after which the secondary piston 3 continues to move alone and is progressively moved deeper into the bore of the barrel section 69. Engagement between the outer sleeve 41 and the abutment 68 results in closing of communication between the entrance 51 of the relief passage 48 and the chamber, and that occurs as the valve slide 9 arrives at or approaches its fully opened position. Thus, build-up of pressure in the chamber 5 is caused by cessation of movement of the piston rod 7 and fluid cannot escape through the relief passage 48 so that the actuator device associated with the conduit 31 is subsequently actuated.

When pressurized fluid is not being fed through either of the conduits 31 or 32 i.e., the actuator is static the control valves 23a and 23b assume closed positions as shown in FIG. 14. Thus, fluid is trapped in both of the chambers 5 and 6 and the escape means previously described enables relief of pressure from those chambers-if the pressure therein becomes excessive because of fluid expanding under increased temperature. If the pressure increase occurs in chamber 5, that increase will be also present in the conduit 15 because of its constant connection with the chamber 5, and the conduit 15 is also in constant communication with safety valve 71,through port 25 and passage 76 as shown in FIG. 15. The valve 71 is arranged to open when the pressure reaches a predetermined limit, and thereby allows exhaust of fluid .to conduit 32 by way of passage 77 and port 29.

If the pressure increase occurs in the chamber 6,

safety valve 72 will open because it is in constant com munication with the chamber 6 through interconnecting passages 24 and 82. Exhaust of fluid from the valve chamber 79 to conduit 31, occurs through passage 83 and port 28 as shown in FIG. 16. I

The construction described may be modified in numerous ways to achieve the basic requirements of the invention. For example, a single relief valve may serve both chambers 5 and 6 of the cylinder 4 if the associated relief passages are appropriately arranged.

Also, the actuator maybe constructed to provide a the relief means and cut-out means are suitably modified. Still further, the stop collar 37 may be contained within or enveloped by the primary piston 2 so as to be capable of only limited axial movement relative thereto. Also, the primary piston 2 may consist of a single sleeve section, in which case the relief means within the piston assembly is arranged to suit.

Yet another embodiment of the invention is shown in FIGS. 18 and 24, and that embodiment is particularly suitable for use with a rotary or semi-rotary valve such as a butterfly valve. For convenience, components of this embodiment which correspond to components of the first described embodiment will be identified by like reference numerals, except that those numerals will form part of the series 101 to 200.

. According to this embodiment, the cylinder 104 contains a piston assembly 101 having a pair of cylindrical end portions 189 which are joined by an intermediate portion 191 as shown in FIG. 20. Each end portion 189 fonns a movable end wall of a respective one of the cylinder chambers 105 and 106, and end caps 111 and 118 are secured to opposite ends of the cylinder 104 substantially as in the previous embodiment. It is preferred in this embodiment however, that a separate valve block 119 be formed integral with or attached to each of the end caps 111 and 118, and each of those blocks 119 contains a control valve 123 as shown in FIG. 21, which is operative to control passage of fluid into and out of the adjacent cylinder chamber. Control valves 123a and 123k are associated with chambers 105 and 106 respectively.

Each control valve 123 preferably includes the same basic components as the valves 23 described in relation to the first embodiment, and functions in the same manner. The principal advantages are that each spool element 127 includes a single stem 135, and the compartments 126 communicate through two conduits 192 and 193 as shown in FIGS. 18 and 21. The conduit 192 connects the front side of compartment 126a to the rear side of compartment l26b so that pressurized fluid entering the compartment 126a from the conduit 131 and through port 128 is effective to cause valve l23b to open through the medium of the spool element 127b and thereby allow the chamber 106 to exhaust through port 129 to conduit 132. The conduit 193 connects the front side of the compartment 126b to the rear side of the compartment 126a to enable both valves 123b and 123a to open when the conduits 132 and 131 function as feed and drain lines respectively.

The valve compartment 122 of valve 123a communicates with the chamber 105 through a passage 125 as shown in FIGS. 21, 23 and 24, and a passage 124 connects the compartment 121 of the valve 123!) to the chamber 106 as also shown in FIG. 21. FIG. 21 shows the valves 123a and 123b positioned as when fluid is fed to the chamber 105 through the conduit 131 and is drained from the chamber 106 through the conduit 132.

Relief means is preferably located externally of the cylinder 104 and in the particular arrangement shown includes a valve block 194 containing relief valves 146 and 147 which function substantially the same as the corresponding valves 46 and 47 of the previously described embodiment. Relief valve 146 includes a member 155 arranged to close an orifice 156 of a relief passage 148 which communicates with the chamber 105 through a passage 150 of the block 194 and interconnected passages 151 and 164 of the adjacent block 1 19, as shown in FIG.23. The valve 146 exhausts to the chamber 106 through a passage 153 and connecting port 195 of the valve block 194, and a conduit 196 (see FIG. 19) which connects the port 195 to interconnected passages 152 and 163 of the other valve block 119 (i.e., that containing valve 123b) as shown in FIG. 19. The passages 152 and 163 communicate direct to the chamber 106 and correspond to passages 151 and 164 respectively of the other block 119. In this regard, the passage arrangement in both of the valve blocks 1 19 is substantially identical.

The relief valve 147 receives high pressure fluid from the chamber 106 through conduit 196, port 195 and a passage 149 of the valve block 194. It is exhausted to the chamber 105 through the interconnected passages 150, 151 and 164. 1

A manually operable by-pass valve 197 is preferably connected to the relief valve assembly to enable the valves 146 and 147 to be by-passed when desired. For example, such by-pass allows manual movement of the piston assembly 101 during maintenance of the actuator. The valve 197 includes a closure member 198 which is operable by a hand wheel 199 to open and close a by-pass orifice 201 which extends between and connects the port 195 and passage 148 as shown in FIG. 22.

Cut-out means for this embodiment preferably includes a pair of stop elements 202 and 203 which contain the passages 164 and 163 respectively. The axially inner end face of the element 202 forms the abutment 1,67 and the corresponding end face of the element 203 forms the abutment 168. Both the elements 202 and 203 may be adjustably mounted within the respective valve blocks 1 19 as shown in FIG. 19, to allow variation of the axial position of the respective end faces 167 and .168, and consequently the two stop positions of the piston assembly 101. v

It will be appreciated from the foregoing that the relief valve 146 is rendered ineffective when an end portion 189 of the piston assembly 101 engages the abutment face 168 of the stop element 203. In that position of the piston assembly 101, the passage 163 is closed and high pressure fluid is unable to escape from the chamber into chamber 106, even though the relief valve 146 may be open and connects the conduit 196 to the chamber 105. Thus, under such conditions, the pressure within chamber 105, and consequently conduit 131, progressively increases until the indicator pressure is reached. During operation of the actuator between the stop positions of the piston assembly, the chamber 106 remains in communication with the relief valve assembly through the passage 163, but as with the previously described embodiment, the relief valve 146 is arranged so as to remain closed under normal operating pressure conditions.

The abutment face 167 and relief valve 147 function in the same manner as described for abutment face 168 and relief valve 146, when pressurized fluid is introduced to the chamber 106 for moving the piston assembly 101 in the opening direction.

Safety means similar to that described in relation to the first embodiment is also preferably provided in each valve block 119. For convenience, only the safety means in the block 119 containing the valve 1230 will be described, and it is to be understood that identical means is provided in the other block 1 19.

Referring to FIGS. 23 and 24, a safety valve 171 includes a valve member 173 contained with a compartment 174 and spring influenced to normally close an orifice 175. The orifice 175 communicates direct with the chamber 105 through passage 125, and the compartment 174 communicates with the port 128 through a passage 177. Thus, when valves 123a and l23b are closed, excessive pressure within the chamber 105 causes the valve 171 to open and exhaust fluid to the conduit 131 through the port 128. The corresponding safety valve of the block 119 containing control valve 123b, functions in the same way .to exhaust chamber 106 to the conduit 132. A

In the" arrangement particularly shown in 1 the drawings, the piston assembly 101 is connectable to the valve element of a butterfly or similar v semi-rotary valve, and that connection may be effected through a rack and pinion. The rack 204 may be formed on the intermediate portion 191 of the piston assembly 101 as shown -in FIG. 20, and the pinion 205 is secured to a rotatable drive shaft 206 adapted to be operatively connected to the butterfly valve. The shaft 206 may be rotatably mounted in a housing 209 formed integral with or connected to the cylinder 104.

A visual indicator may be associated with the actuator in addition to an indicator device of the kind described in relation to the previous embodiment. Such an additional indicator may include a pointer 207 secured to the shaft 206, and a scale 208 secured to the cylinder 104, as shown in FIG. 19. In FIG. 19 the indicator shows the piston assembly 101 to be in its intermediate position in which a connected valve member is located between its open and closed positions.

It will be clear that an actuator construction according to FIGS. 18 to 24, may be connected into a fluid system to operate in basically the same manner as the construction first described. That is, in operation the actuator of FIGS. 18 to 24 may be included in a fluid circuit as shown in FIG. 17. If desired, the two piston end portions 189 may be of a different size to produce a difference in the closing and opening forces for the same fluid pressure.

The basic actuator construction last described may be also applied to linear movement valves, but for such an application it may be more convenient to have each piston end portion 189 contained within respective spaced cylinders rather than a single cylinder as shown and described. Any of the relief and cut-out means described may be used with such a construction, and the connection of each piston end portion 189 to the valve slide may take any convenient form.

Although the specific embodiments of the actuator described have included a piston or pistons as the force transmitting element of the actuator, it is to be understood that other types of elements may be used. For example, the force transmitting element may be a diaphragm, bellows, or any other element adapted to translate fluid pressure into mechanical movement.

It will be appreciated that an actuator as described has considerable potential because of its simple yet effective construct-ion. In a marine application as described for example, the expense and complexity of the actuator and indicator installations are both significantly reduced. Furthermore, the actuator is not susceptible to giving false indications because of leakage in the fluid system, and it is substantially failsafe in that all valves return to a closed position if the fluid system fails.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the preferred constructions and arrangements of parts previously described without departing from the spirit or'ambit of the invention.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

l. A fluid operated actuator including an actuator element movable under the influence of fluid pressure and being connectable to a member to be actuated, pressure relief means operable to prevent the pressure acting on said actuator element reaching a predetermined level, and cut-out means operable to render said relief means inoperative when said actuator element adopts a particular axial position, said actuator element being defined by a piston assembly which is slidable axially within a cylinder'and divides theinterior of that cylinder. transversely into two chambers, and said pressure relief means communicates with at least one of said chambers when said cut-out means is inoperative, said cut-out means including valve means forming at least part of said piston assembly for closing communication between said'one chamber and said relief means when said actuator element is in said particular axial position. I

2. An actuator according to claim 1, wherein said cylinder includes inlet means for both said chambers whereby pressurized fluid may be introduced to either of said chambers according to the desired direction of movement of the piston assembly; and said pressure relief means is operable to relieve pressure from whichever of said chambers is subjected to said pressurized fluid, so long as said cut-out means remains inoperative.

3. An actuator according to claim 1, wherein said relief means includes at least one relief valve.-

4. An actuator according to claim 2, wherein said relief means includes two relief valves, each of which is operatively connected to a respective one of said chambers.

5. An actuator according to claim 1, wherein said piston assembly has a first said particular operative position in which said part thereof closes communication between one of said chambers and said pressure relief means, and is positioned adjacent the end of the cylinder remote from that chamber; and a second said particular axial position in which said part thereof closes communication between the other said chamber, and is positioned adjacent the end of said cylinder opposite said firstmentioned end thereof.

6. An actuator according to claim 1, wherein means is provided to reduce the efiective end surface area of the piston assembly to which pressurized fluid is applied to cause movement of the piston assembly in one direction, said means being arranged to cause said area piston assembly has a first said particular operative position in which a first part of said piston assembly closes communication between one of said chambers and said pressure relief means, and is positioned adjacent the end of the cylinder remote from that chamber; and a second said particular operative position in which a second part of said piston assembly closes communication between the other said chamber and said pressure relief means, and is positioned adjacent the end of said cylinder opposite said firstmentioned end thereof.

8. A fluid operated actuator including, a cylinder, a piston assembly slidable axially within said cylinder and dividing the interior thereof transversely to define two variable volume chambers,'inlet means to allow introduction of pressurized fluid to one of said chambers, connecting means projecting through a wall of said cylinder for connecting said piston assembly to a member to be actuated, pressure relief means connected to said one chamber and being operable to prevent the fluid pressure therein reaching a predetermined level, and cut-out means operable to render the relief means inoperative when the piston assembly adopts a particular axial position relative to said cylinder, said cut-out means including valve means forming part of said piston assembly for closing communication between said one chamber and said pressure relief means when said piston assembly adopts said particular axial position. v

9. An actuator according to claim 8, wherein fluid inlet and outlet means is provided for both said chambers so that pressurized fluid may be introduced to either of said chambers according to the desired direction of movement of the piston assembly, and said pressure relief means is connected to both said chambers to control the pressure within whichever of said chambers is subjected to said pressurized fluid.

10. An actuator according to claim 9, wherein a single port defines both the inlet means and the outlet means for each said chamber, said ports communicating with their respective chambers through a pressure operated control valve arranged to open both said ports to their respective chambers in response to pressurized fluid being fed into either one of said ports, whereby the port receiving said pressurized fluid functions as inlet means for its respective chamber and the other said port functions as outlet means for its chamber.

11. An actuator according to claim 10, wherein said control valve has biasing means to normally close communication between both said ports and their respective chambers, and safety valve means is connected to both said chambers to prevent the pressure therein exceeding a predetermined upper limit.

12. An actuator according to claim 8, wherein said pressure relief means includes a relief valve operatively connected to at least one of said chambers.

13. An actuator according to claim 8, wherein said pressure relief means includes two relief valves, each of which is operatively connected to a respective one of said chambers. 1

14. An actuator according to claim 13, wherein each said relief valve is contained within said piston assembly.

15. An actuator according to claim 8, wherein said piston assembly includes a secondary piston which is connected to an end portion of a piston rod which forms at least part of said connecting means, and a primary piston which is slidably mounted on said secondary piston for limited axial movement relative thereto; said piston rod projecting through an end wall of said cylinder and being movable axially relative to the cylinder with said secondary piston; said pressure relief means includes a passage formed through said secondary piston to provide communication between said chambers; and said valve means includes at least part of said primary piston which-is arranged to open or close said communication according to its axial position relative to said secondary piston.

16. An actuator according to claim 13, wherein said relief means passage has an entrance portion which extends laterally through the surface of said secondary piston'over which said primary piston slides, said primary piston part being located over said entrance por-. tion and closing same when in said particular axial position.

17.- An actuator according to claim 16, wherein; said primary piston includes an inner sleeve section slidable on said secondary piston, and an outer sleeve section which is slidable on said inner sleeve section; stop means is provided on said secondary piston to'limit axial movement of said inner sleeve section relative to the secondary piston; and said outer sleeve section is located between opposed annular shoulders of said inner sleeve section and is engageable therewith to limit its axial movement relative to said inner sleeve section.

18. An actuator according to claim 17, wherein an opening extends radially through said inner sleeve section and remains in communication with said passage entrance portion in all axial positions of said inner sleeve section relative to said secondary piston, and communication between said entrance portion and the adjacent cylinder chamber is closed by engagement between said outer sleeve section and one of said annular shoulders.

19. An actuator according to claim 17, wherein an abutment is provided within said cylinder for engagement by said outer sleeve section to limit axial movement of said outer sleeve section relative to the cylinder in one direction; and said inner sleeve section and secondary piston are able to move further in said one direction after said outer sleeve section and said abutment engage, whereby said outer sleeve section is located relative to said secondary piston to close said relief passage.

20. An actuator according to claim 19, wherein said relief passage is closed when said outer sleeve section engages both the said abutment and the said annular shoulder located remotefrom said abutment; and in that position of the piston assembly the effective end area thereof which is engageable by pressurized fluid to move the said secondary piston further in said one direction, is smaller than the effective opposite end area of the piston assembly which is engageable by pressurized fluid to move said assembly in the direction opposite to said one direction.

21. An actuator according to claim 15, wherein said relief means includes a one-way valve located in said relief passage.

22. An actuator according to claim 15, wherein two said passages are formed through said secondary piston, and said primary piston part is movable axially relative to the secondary piston between two positions in each of which said part closes a respective said passage.

.23. An actuator according to claim 17, wherein two said passages are formed through said secondary piston, said primary piston part is movable axially relative to the secondary piston between two positions in each of which said part closes a respective said passage, two openings extend radially through said inner sleeve section and each remains in communication with the entrance portion of a respective one of said passages in all axial positions of said inner sleeve section relative to said secondary piston, and communication between each said entrance portion and the adjacent cylinder chamber is closed by engagement between said outer sleeve section and a respective one of said annular shoulders.

24. An actuator according to claim 18, wherein two said passages are formed through said secondary piston; said primary piston part is movable axially relative to the secondary piston between two positions in each of which said part closes a respective said passage; a pair of spaced abutments are provided within said cylinder for engagement by said outer sleeve section to limit axial movement of said outer sleeve section relative to the cylinder in two opposite directions; and said inner sleeve section and secondary piston are able to move further in each said direction after said outer sleeve section and the respective said abutment engage, whereby said outer sleeve section is located relative to said secondary piston to close the respective said relief passage.

25. An actuator according to claim 24, wherein said outer sleeve section engages a respective said abutment in each of said two positions, and a reduction in the said effective end area of the piston assembly occurs in each of said positions, the said end area reduced being opposite for each of said positions.

26. An actuator according to claim 22, wherein said relief means includes a one-way relief valve located in each said passage, and said valves are arranged to allow passage of fluid in respective opposite directions.

27. An actuator according to claim 15, wherein a combined inlet and outlet port is provided for each said chamber, each said port communicating with its respective said chamber through a pressure-operated control valve arranged to open both said ports to their respective chambers in response to pressurized fluid being fed into either one of said ports, whereby the port receiving said pressurized fluid functions as inlet means for its respective chamber and the other said port functions as outlet means for its chamber.

28. An actuator according to claim 27, wherein said control valve has biasing means to normally close communication between both said ports and their respective chambers, and safety valve means is connected to both said chambers to prevent the pressure therein exceeding a predetermined upper limit.

29. An actuator according to claim 27, wherein said control valve is contained in a valve block secured to one end of said cylinder; said block including two valve compartments, one of which communicates directly with the adjacent said cylinder chamber, and the other communicates with the remote said cylinder chamber through a conduit; and each said port is connected to a respective said valve compartment when said control valve is opened.

30. An actuator according to claim 29, wherein said control valve includes two one-way valve elements which are biased by spring means towards each other into closed positions in which each closes communication between a respective said port and jvalve compartment, and an axially slidable spool element disposed between said valve elements and providing a fluid seal between said ports; said spool being operative to open one saidvalve element in response to fluid pressure in the port of the other said valve element.

31. An actuator according to claim 8, wherein: said piston assembly includes two cylindrical end portions, an intermediate portion joining said end portions, and a rack formed on said intermediate portion and drivably engageable with a pinion projecting through an aperture in a side wall of said cylinder, said rack and pinion forming at least part of said connecting means; said relief means includes a main relief passage providing a communication between said chambers; and said valve means includes at least one of said end portions of the piston assembly which is arranged to open or close said communication between said chambers according to the axial position of the piston assembly relative to said cylinder.

32. An actuator according to claim 31, wherein said main relief passage is defined by a conduit located externally of said cylinder and which communicates with each said chamber through a respective passage ex tending axially of the cylinder and arranged to be closed by the adjacent end surface of the piston assembly. v

33. An actuator according to claim 32, wherein each said axial passage is formed in a stop element which projects axially into said cylinder from a respective end wall thereof, the end surface of each stop element which is located within said cylinder serving as an abutment for the adjacent end of said piston assembly and having the respective said axial passage opening therethrough.

34. An actuator according to claim 33, wherein each said stop element is adjustably mounted in the respective cylinder end wall to enable variation of the axial position of the said end surface thereof.

35. An actuator according to claim 31, wherein said relief means include relief valve means connected to said main relief passage and operable to open and close communication between said chambers through said main passage, according to the fluid pressure existing in said chambers.

36. An actuator according to claim 35, wherein said relief valve means includes a valve housing having a compartment therein, said main passage communicating direct with said compartment and with one of said cylinder chambers, and compartment inlet and outlet 

1. A fluid operated actuator including an actuator element movable under the influence of fluid pressure and being connectable to a member to be actuated, pressure relief means operable to prevent the pressure acting on said actuator element reaching a predetermined level, and cut-out means operable to render said relief means inoperative when said actuator element adopts a particular axial position, said actuator element being defined by a piston assembly which is slidable axially within a cylinder and divides the interior of that cylinder trAnsversely into two chambers, and said pressure relief means communicates with at least one of said chambers when said cut-out means is inoperative, said cut-out means including valve means forming at least part of said piston assembly for closing communication between said one chamber and said relief means when said actuator element is in said particular axial position.
 2. An actuator according to claim 1, wherein said cylinder includes inlet means for both said chambers whereby pressurized fluid may be introduced to either of said chambers according to the desired direction of movement of the piston assembly; and said pressure relief means is operable to relieve pressure from whichever of said chambers is subjected to said pressurized fluid, so long as said cut-out means remains inoperative.
 3. An actuator according to claim 1, wherein said relief means includes at least one relief valve.
 4. An actuator according to claim 2, wherein said relief means includes two relief valves, each of which is operatively connected to a respective one of said chambers.
 5. An actuator according to claim 1, wherein said piston assembly has a first said particular operative position in which said part thereof closes communication between one of said chambers and said pressure relief means, and is positioned adjacent the end of the cylinder remote from that chamber; and a second said particular axial position in which said part thereof closes communication between the other said chamber, and is positioned adjacent the end of said cylinder opposite said firstmentioned end thereof.
 6. An actuator according to claim 1, wherein means is provided to reduce the effective end surface area of the piston assembly to which pressurized fluid is applied to cause movement of the piston assembly in one direction, said means being arranged to cause said area reduction when said piston assembly is at or adjacent the said particular axial position; and the opposite effective end surface area of the piston assembly in which pressurized fluid is applied for initially moving the piston assembly in the opposite direction, is greater than said reduced area.
 7. An actuator according to claim 1, wherein said piston assembly has a first said particular operative position in which a first part of said piston assembly closes communication between one of said chambers and said pressure relief means, and is positioned adjacent the end of the cylinder remote from that chamber; and a second said particular operative position in which a second part of said piston assembly closes communication between the other said chamber and said pressure relief means, and is positioned adjacent the end of said cylinder opposite said firstmentioned end thereof.
 8. A fluid operated actuator including, a cylinder, a piston assembly slidable axially within said cylinder and dividing the interior thereof transversely to define two variable volume chambers, inlet means to allow introduction of pressurized fluid to one of said chambers, connecting means projecting through a wall of said cylinder for connecting said piston assembly to a member to be actuated, pressure relief means connected to said one chamber and being operable to prevent the fluid pressure therein reaching a predetermined level, and cut-out means operable to render the relief means inoperative when the piston assembly adopts a particular axial position relative to said cylinder, said cut-out means including valve means forming part of said piston assembly for closing communication between said one chamber and said pressure relief means when said piston assembly adopts said particular axial position.
 9. An actuator according to claim 8, wherein fluid inlet and outlet means is provided for both said chambers so that pressurized fluid may be introduced to either of said chambers according to the desired direction of movement of the piston assembly, and said pressure relief means is connected to both said chambers to control the pressure within whichever of said cHambers is subjected to said pressurized fluid.
 10. An actuator according to claim 9, wherein a single port defines both the inlet means and the outlet means for each said chamber, said ports communicating with their respective chambers through a pressure operated control valve arranged to open both said ports to their respective chambers in response to pressurized fluid being fed into either one of said ports, whereby the port receiving said pressurized fluid functions as inlet means for its respective chamber and the other said port functions as outlet means for its chamber.
 11. An actuator according to claim 10, wherein said control valve has biasing means to normally close communication between both said ports and their respective chambers, and safety valve means is connected to both said chambers to prevent the pressure therein exceeding a predetermined upper limit.
 12. An actuator according to claim 8, wherein said pressure relief means includes a relief valve operatively connected to at least one of said chambers.
 13. An actuator according to claim 8, wherein said pressure relief means includes two relief valves, each of which is operatively connected to a respective one of said chambers.
 14. An actuator according to claim 13, wherein each said relief valve is contained within said piston assembly.
 15. An actuator according to claim 8, wherein said piston assembly includes a secondary piston which is connected to an end portion of a piston rod which forms at least part of said connecting means, and a primary piston which is slidably mounted on said secondary piston for limited axial movement relative thereto; said piston rod projecting through an end wall of said cylinder and being movable axially relative to the cylinder with said secondary piston; said pressure relief means includes a passage formed through said secondary piston to provide communication between said chambers; and said valve means includes at least part of said primary piston which is arranged to open or close said communication according to its axial position relative to said secondary piston.
 16. An actuator according to claim 13, wherein said relief means passage has an entrance portion which extends laterally through the surface of said secondary piston over which said primary piston slides, said primary piston part being located over said entrance portion and closing same when in said particular axial position.
 17. An actuator according to claim 16, wherein; said primary piston includes an inner sleeve section slidable on said secondary piston, and an outer sleeve section which is slidable on said inner sleeve section; stop means is provided on said secondary piston to limit axial movement of said inner sleeve section relative to the secondary piston; and said outer sleeve section is located between opposed annular shoulders of said inner sleeve section and is engageable therewith to limit its axial movement relative to said inner sleeve section.
 18. An actuator according to claim 17, wherein an opening extends radially through said inner sleeve section and remains in communication with said passage entrance portion in all axial positions of said inner sleeve section relative to said secondary piston, and communication between said entrance portion and the adjacent cylinder chamber is closed by engagement between said outer sleeve section and one of said annular shoulders.
 19. An actuator according to claim 17, wherein an abutment is provided within said cylinder for engagement by said outer sleeve section to limit axial movement of said outer sleeve section relative to the cylinder in one direction; and said inner sleeve section and secondary piston are able to move further in said one direction after said outer sleeve section and said abutment engage, whereby said outer sleeve section is located relative to said secondary piston to close said relief passage.
 20. An actuator according to claim 19, wherein said relief passage is closed when said outer sleeve sectIon engages both the said abutment and the said annular shoulder located remote from said abutment; and in that position of the piston assembly the effective end area thereof which is engageable by pressurized fluid to move the said secondary piston further in said one direction, is smaller than the effective opposite end area of the piston assembly which is engageable by pressurized fluid to move said assembly in the direction opposite to said one direction.
 21. An actuator according to claim 15, wherein said relief means includes a one-way valve located in said relief passage.
 22. An actuator according to claim 15, wherein two said passages are formed through said secondary piston, and said primary piston part is movable axially relative to the secondary piston between two positions in each of which said part closes a respective said passage.
 23. An actuator according to claim 17, wherein two said passages are formed through said secondary piston, said primary piston part is movable axially relative to the secondary piston between two positions in each of which said part closes a respective said passage, two openings extend radially through said inner sleeve section and each remains in communication with the entrance portion of a respective one of said passages in all axial positions of said inner sleeve section relative to said secondary piston, and communication between each said entrance portion and the adjacent cylinder chamber is closed by engagement between said outer sleeve section and a respective one of said annular shoulders.
 24. An actuator according to claim 18, wherein two said passages are formed through said secondary piston; said primary piston part is movable axially relative to the secondary piston between two positions in each of which said part closes a respective said passage; a pair of spaced abutments are provided within said cylinder for engagement by said outer sleeve section to limit axial movement of said outer sleeve section relative to the cylinder in two opposite directions; and said inner sleeve section and secondary piston are able to move further in each said direction after said outer sleeve section and the respective said abutment engage, whereby said outer sleeve section is located relative to said secondary piston to close the respective said relief passage.
 25. An actuator according to claim 24, wherein said outer sleeve section engages a respective said abutment in each of said two positions, and a reduction in the said effective end area of the piston assembly occurs in each of said positions, the said end area reduced being opposite for each of said positions.
 26. An actuator according to claim 22, wherein said relief means includes a one-way relief valve located in each said passage, and said valves are arranged to allow passage of fluid in respective opposite directions.
 27. An actuator according to claim 15, wherein a combined inlet and outlet port is provided for each said chamber, each said port communicating with its respective said chamber through a pressure-operated control valve arranged to open both said ports to their respective chambers in response to pressurized fluid being fed into either one of said ports, whereby the port receiving said pressurized fluid functions as inlet means for its respective chamber and the other said port functions as outlet means for its chamber.
 28. An actuator according to claim 27, wherein said control valve has biasing means to normally close communication between both said ports and their respective chambers, and safety valve means is connected to both said chambers to prevent the pressure therein exceeding a predetermined upper limit.
 29. An actuator according to claim 27, wherein said control valve is contained in a valve block secured to one end of said cylinder; said block including two valve compartments, one of which communicates directly with the adjacent said cylinder chamber, and the other communicates with the remote said cylinder chamber through a coNduit; and each said port is connected to a respective said valve compartment when said control valve is opened.
 30. An actuator according to claim 29, wherein said control valve includes two one-way valve elements which are biased by spring means towards each other into closed positions in which each closes communication between a respective said port and valve compartment, and an axially slidable spool element disposed between said valve elements and providing a fluid seal between said ports; said spool being operative to open one said valve element in response to fluid pressure in the port of the other said valve element.
 31. An actuator according to claim 8, wherein: said piston assembly includes two cylindrical end portions, an intermediate portion joining said end portions, and a rack formed on said intermediate portion and drivably engageable with a pinion projecting through an aperture in a side wall of said cylinder, said rack and pinion forming at least part of said connecting means; said relief means includes a main relief passage providing a communication between said chambers; and said valve means includes at least one of said end portions of the piston assembly which is arranged to open or close said communication between said chambers according to the axial position of the piston assembly relative to said cylinder.
 32. An actuator according to claim 31, wherein said main relief passage is defined by a conduit located externally of said cylinder and which communicates with each said chamber through a respective passage extending axially of the cylinder and arranged to be closed by the adjacent end surface of the piston assembly.
 33. An actuator according to claim 32, wherein each said axial passage is formed in a stop element which projects axially into said cylinder from a respective end wall thereof, the end surface of each stop element which is located within said cylinder serving as an abutment for the adjacent end of said piston assembly and having the respective said axial passage opening therethrough.
 34. An actuator according to claim 33, wherein each said stop element is adjustably mounted in the respective cylinder end wall to enable variation of the axial position of the said end surface thereof.
 35. An actuator according to claim 31, wherein said relief means include relief valve means connected to said main relief passage and operable to open and close communication between said chambers through said main passage, according to the fluid pressure existing in said chambers.
 36. An actuator according to claim 35, wherein said relief valve means includes a valve housing having a compartment therein, said main passage communicating direct with said compartment and with one of said cylinder chambers, and compartment inlet and outlet passages each of which communicates with the other said cylinder chamber and is controlled by a respective one-way valve, the two said one-way valves being arranged to respectively allow fluid into and out of said compartment.
 37. An actuator according to claim 32, wherein said relief means include relief valve means connected to said main relief passage and operable to open and close communication between said chambers through said main passage, according to the fluid pressure existing in said chambers; said relief valve means including a valve housing having a compartment therein, said main passage communicating direct with said compartment and with one of said cylinder chambers, and compartment inlet and outlet passages each of which communicates with the other said cylinder chamber and is controlled by a respective one-way valve, the two said one-way valves being arranged to respectively allow fluid into and out of said compartment, said compartment inlet passage communicating with one of said axial passages, and said conduit communicating direct with the other said axial passage.
 38. An actuator according to claim 37, wherein said relief valve means includes a manually operable by-pass valve whIch is operable to open direct communication between said conduit and said one axial passage.
 39. An actuator according to claim 31, wherein each of the opposite end walls of said cylinder is defined by a valve block which includes: a combined fluid supply and exhaust port, a pressure operable one-way valve which is normally biased into a closed position and controls fluid flow through said port, and an axially slidable spool element which is pressure operable to open said one-way valve; and respective drain passage means connects the said port of each said valve block to a rear side of the said spool element of the other said valve block, whereby pressurized fluid supplied to one said chamber through the adjacent said valve block is operative to cause the said spool element of the other said valve block to open its respective said one-way valve and thereby allow said other chamber to exhaust. 40 An actuator according to claim 39, wherein safety valve means is provided within each said valve block and communicates with the adjacent said chamber to prevent the pressure therein exceeding a predetermined upper limit. 